1. FIELD OF THE INVENTION
The invention relates to signalling circuit failure when the variable sensed generates its minimum signal in its normal range. More specifically, in a level detection circuit responding to radiation, the invention relates to low-level detection and signal of circuit failure during the period of normal level.
2. DESCRIPTION OF THE PRIOR ART
Circuits which actuate a relay when responding to a specified condition have several problems in providing a fail-safe mode of operation. It is well to consider level detection as an example. Further, those level detection circuits built to utilize radiation give a more specific example.
The more simple of the examples is that where a source of radiation is directed across a container of liquid in line with a detector of the radiation. When the level of the liquid lowers to where the radiation passes above the body of the liquid and is received at the detector, the circuit of the detector is activated to throw a relay in signal of the low level. Simple.
The problem of the prior art is that with the normal mode of operation, when the level is above the line of sight between the source and detector, the detector circuit is passive. The circuit is receiving the minimum normal input, or no input. Then the question descends. How is a failure of the circuit signalled?The circuit is passive during the normal level of the liquid. How are we to tell when it fails during this period? The prior art had no answer I was able to find.